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Sticking together Social monogamy developed in primates as a way for males to protect their offspring from infanticide, new research has found, although this may not explain the rise of monogamy in mammals as a whole.

"When you get higher rates of infanticide, you tend to get social monogamy arising," says Atkinson.

A number of hypotheses have been established to explain the evolution of social monogamy, in which males and females form a pair and raise offspring together, he says.

The 'mate-guarding' hypothesis is based on the idea that males need to keep tabs on females if they are to make sure they don't mate with other males.

When those females are spread out over a large area, monogamy might the best strategy to do this.

"It becomes the only option for males to ensure that the children of the female they're with are their own," says Atkinson.

The 'biparental' hypothesis suggests that having two parents looking after the young means a more reliable food supply and healthier infants.

And a third more gruesome hypothesis is that monogamy is a defence against infanticide.

When male animals find infants that are not theirs, they will kill them because they're taking up resources their infants could have, says Atkinson.

By killing the offspring of another male that means that the mother of that offspring is freed up from suckling and can become pregnant with the infanticidal male's offspring.

But to protect their own offspring against infanticide, males need to identify which offspring are theirs, says Atkinson. Being monogamous provides a strategy for doing this.

Testing between hypotheses

To test between these three hypotheses, Atkinson and colleagues studied 230 different species of primates including humans.

First, gene-based phylogenies allowed the researchers to map the relationship between species.

Then, by studying social traits in species that are alive today, the researchers were also able to calculate when these first evolved.

The three social traits studied related to the three different hypotheses on the evolution of monogamy.

The researchers looked at rates of infanticide in the group and whether the species were living in pairs (relevant to the biparental care hypothesis).

They also looked at whether the females lived in a group in one place or by themselves, spread over a large region. This was relevant to the mate-guarding hypothesis, since males are forced to guard just one female if they are spread out over a large distance.

"We found that the only thing that cropped up regularly before you get the emergence of social monogamy is a high level of infanticide," says Atkinson.

"That's the evidence we use to argue the risk of infanticide might have been what drove social monogamy in primates."

The researchers say that social monogamy allowed fathers to become involved in parental care, and is associated with a shift in the range of animals.

Mammalian monogamy study

Meanwhile, a related study in the journal Science this week looking at the rise of social monogamy in mammals as a whole, has come to a different conclusion.

Zoologists Dr Dieter Lukas and Professor Tim Clutton-Brock from the University of Cambridge used data on more than 2,500 mammalian species and found support for the mate-guarding hypothesis, rather than the infanticide hypothesis.

"The evolution of social monogamy does not appear to have been associated with a high risk of male infanticide," write the researchers.

"Social monogamy has evolved in non-human mammals where breeding females are intolerant of each other and female density is low, suggesting that it represents a mating strategy that has developed where males are unable to defend access to multiple females."

Atkinson says the different findings could be explained by the fact that the Cambridge study looked at the whole group of mammals whereas his study looked at primates.

"Less than three per cent of mammals are monogamous; it's much more common in primates," says Atkinson.

"So there is good reason to think that something different is going on in primates."

Further studies

Australian evolutionary biologist Professor Rob Brooks from the University of New South Wales says the studies are dealing with different samples and the mammal study is a "blunter analysis".

But at any rate, he says, the cause and effect relationships inferred from these studies need further investigation.

Brooks also emphasises that the findings have little relevance to the human obsession with understanding faithfulness and cheating.

"That's where the hook is for people and it's a long way from that hook," he says.

"This isn't about that dynamic it all. It's about why it is we find one person compelling and attractive enough to stay with them for a long period of time."